Apache Rewrite to Nginx with ProxyPass

I'm trying to redirect an Subdomain hosted on an Apache Webserver* with .htaccess to an external URL with hiding this external URL. The .htaccess file looks like this:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^sub.domain\.com [NC]
RewriteRule ^(.*) http://external-domain.com/$1 [P]

How I must configure the Nginx site? The site is very simple and works if called directly in the local network:

server {
    listen 80;

    server_name sub.domain.com;

    root /usr/share/nginx/html/sub.domain.com;
    index index.html;
}

I guess there must be some ProxyPass magic, but I cannot figure out what. It must be something like

location ^~ / {    
    proxy_pass http://localhost;
    proxy_set_header X-Real-IP 127.0.0.1;
    proxy_set_header Host sub.domain.com;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;
}

but this does not work. I would appreciate any help. Thanks...

*The Webserver is hosted by an simple Provider, no special configurations can be made

I took the simplest solution. I have deleted the subdomain and added an CNAME DNS Record. That works.